1. Field of the Invention
The present invention relates generally to methods and apparatus for reproducing images and alphanumeric characters, more particularly to ink-jet hard copy apparatus and, more specifically to a thermal ink-jet, multi-orifice drop generator, print head construct and its method of operation.
2. Description of Related Art
The art of ink-jet hard copy technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and, Vol. 45, No.1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy Devices, chapter 13 (Ed. R. C. Durbeck and S. Sherr, Academic Press, San Diego, 1988).
It has been estimated that the human visual system can distinguish ten million colors. Printing systems use a small subset of colors, yet can create acceptable reproductions of original images. Generally speaking, this is achieved by mixing the primary colors (red, blue green-additive; or cyan, magenta, yellow-subtractive) in sufficiently small quanta and exploiting tristimulus response idiosyncrasies of the human visual system. Effective use of these small quanta can be achieved in dot matrix color printing by varying the density or area fill, or both, to recreate each color or a reasonable semblance thereof in the image.
The quality of a printed image has many aspects. When the printed matter is an image that is a reproduction of an original image (that is to say, a photograph or graphic design rather than merely text printing), the goal of an imaging system is to accurately reproduce the appearance of the original. To achieve this goal, the system must accurately reproduce both the perceived colors (hues) and the perceived relative luminance ratios (tones) of the original. Human visual perception quickly adjusts to wide variations in luminance levels, from dark shadows to bright highlights. Between these extremes, perception tends toward an expectation of smooth transitions in luminance. However, imaging systems have yet to achieve complete faithful reproduction of the full dynamic range and perception continuity of the human visual system. While the goal is to achieve true photographic image quality reproduction, imaging systems"" dynamic range printing capabilities are limited by the sensitivity and saturation level limitations inherent to the recording mechanism. The effective dynamic range can be extended somewhat by utilizing a non-linear conversion that allows some shadow and highlight detail to remain.
In ink-jet technology, which uses dot matrix manipulation to form both images and alphanumeric characters, the colors and tone of a printed image are modulated by the presence or absence of drops of ink deposited on the print medium at each target picture element (known as xe2x80x9cpixelsxe2x80x9d) of a superimposed rectangular grid overlay of the image. The luminance continuityxe2x80x94tonal transitions within the recorded imagexe2x80x94is especially affected by the inherent quantization effects of using ink droplets and dot matrix imaging. These effects can appear as contouring in printed images where the original image had smooth transitions. Moreover the imaging system can introduce random or systematic luminance fluctuations (graininessxe2x80x94the visual recognition of individual dots with the naked eye).
Perceived quantization effects which detract from print quality can be reduced by decreasing the physical quantization levels in the imaging system and by utilizing techniques that exploit the psycho-physical characteristics of the human visual system to minimize the human perception of the quantization effects. It has been estimated that the unaided human visual system will perceive individual dots until they have been reduced to less than or equal to approximately twenty to twenty-five microns in diameter in the printed image. Therefore, undesirable quantization effects of the dot matrix printing method are reduced in the current state of the art by decreasing the size of each drop and printing at a high resolution; that is, a 1200 dots per inch (xe2x80x9cdpixe2x80x9d) printed image looks better to the eye than a 600 dpi image which in turn improves upon 300 dpi, etc. Additionally, undesired quantization effect can be reduced by utilizing more pen colors with varying densities of color (e.g., two cyan ink print cartridges, each containing a different dye load (the ratio of dye to solvent in the chemical composition of the ink) or containing different types of chemical colorants, dye-based or pigment-based).
To reduce quantization effects, print quality also can be enhanced by methods of saturating each pixel with large volumes of dye by using large drops, a high dye-load ink formula, or by firing multiple drops of the same color or color formulation at each pixel. Such methods are discussed in U.S. Pat. No. 4,967,203 (Doan) for an Interlace Printing Process, U.S. Pat. No. 4,999,646 (Trask) for a Method for Enhancing the Uniformity and Consistency of Dot Formation Produced by Color Ink Jet Printing, and U.S. Pat. No. 5,583,550 (Hickman) for Ink Drop Placement for Improved Imaging (each assigned to the common assignee of the present invention). However, large drops create large dots, or larger groups of dots known as xe2x80x9csuperpixels,xe2x80x9d which are quite visible in transition zones. Moreover, each of these methods consume ink at a rapid rate and are thus more expensive to operate. Drop volume control and multi-drop methods of inking are taught respectively by Childers in U.S. Pat. No. 4,967,208 for an Offset Nozzle Droplet Formation and U.S. Pat. No. 5,485,180 (Askeland et al.) for Inking for Color-Inkjet Printers, Using Non-Integral Drop Averages, Media Varying Inking, or More Than Two Drops Per Pixel (each assigned to the common assignee of the present invention). In a multi-drop mode, the resulting dot will vary in size or in color depending on the number of drops fired at an individual pixel or superpixel and the constitution of the ink with respect to its spreading characteristics after impact on the particular medium being printed (plain paper, glossy paper, transparency, etc.). The luminance and color of the printed image is modulated by manipulating the size and densities of drops of each color at each target pixel. The quantization effects of this mode can be physically reduced in the same ways as for the single-drop per pixel mode. The quantization levels can also be reduced at the same printing resolution by increasing the number of drops that can be fired at one time from each nozzle in a print head array and either adjusting the density of the ink or the size of each drop fired so as to achieve full dot density. However, simultaneously decreasing drop size and increasing the printing resolution, or increasing the number of pens and varieties of inks employed in a hard copy apparatus is very expensive, so ink-jet hard copy apparatus designed specifically for imaging art reproduction generally use multi-drop modes to improve color saturation.
When the size of the printed dots is modulated the image quality is very dependent on dot placement accuracy and resolution. Misplaced dots leave unmarked pixels which appear as white dots or even bands of white lines within or between print swaths (known as xe2x80x9cbandingxe2x80x9d). Mechanical tolerances are critical in the construction as the print head geometries of the nozzles are reduced in order to achieve a resolution of 600 dpi or greater. Therefore, the cost of manufacture increases with the increase of the resolution design specification. Furthermore, as the number of drops fired at one time by multiplexing nozzles increases, the minimum nozzle drop volume decreases, dot placement precision requirements increase, and thermal efficiency of the print head becomes more difficult to control. High temperatures not only bum out print head elements faster but also have to be taken into account when formulating the inks to be used.
When the density of the printed dots is modulated, the low dye load inks require that more ink be placed on the print media, resulting in less efficient ink usage and higher risk of ink coalescence and smearing. Ink usage efficiency decreases and risk of coalescence and smearing increases with the number of drops fired at one time from each nozzle of the print head array.
Another methodology for controlling print quality is to focus on the properties of the ink itself. When an ink drop contacts the print media, lateral diffusion (xe2x80x9cspreadingxe2x80x9d) begins, eventually ceasing as the colorant vehicle (water or some other solvent) of the ink is sufficiently spread and evaporates. For example, in U.S. Pat. No. 4,914,451 (Morris et al., assigned to the common assignee of the present invention), Post-Pointing Image Development of Ink-Jet Generated Transparencies, lateral spreading of each droplet is controlled with media coatings that control latent lateral diffusion of the printed ink dots. However, this increases the cost of the print media. Lateral spreading also causes adjacent droplets to bleed into each other. The ink composition itself can be constituted to reduce bleed, such as taught by Prasad in U.S. Pat. No. 5,196,056 for an Ink Jet Composition with Reduced Bleed. However, this may result in a formulation not suitable for the spectrum of available print media that end users may find desirous.
One apparatus for improving print quality is discussed in a very short article, Bubble Ink-Jet Technology with Improved Performance, by Enrico Manini, Olivetti, presented at ISandT""s Tenth International Congress on Advances in Non-Impact Printing Technologies, Oct. 30-Nov. 4, 1994, New Orleans, La. Manini shows a concept for, xe2x80x9cbetter distributing the ink on the paper, by using more, smaller droplets . . . utiliz(ing) several nozzles for each pressure chamber, so that a fine shower of ink is deposited on the paper.xe2x80x9d Sketches are provided by Manini showing two-nozzle pressure chambers, three-nozzle chambers, and four-nozzle chambers. Manini shows the deposition of multiple drops of ink within a pixel areal dimension such that individual drops are in adjacent contact or overlapping. Manini alleges the devices abilities: to make a square elementary dot to thereby provide a 15% ink savings and faster drying time; to create better linearity in gray scaling; and to allow the use of smaller nozzles which allow higher capillary refill (meaning a faster throughput capabilityxe2x80x94generally measured in printed pages per minute, xe2x80x9cppmxe2x80x9d). No working embodiment is disclosed and Manini himself admits, xe2x80x9cThe hydraulic tuning between the entrance duct and the outlet nozzles is however rather complex and requires a lot of experimentation.xe2x80x9d
Manini, however, only followed along the path of prior U.S. Pat. No. 4,621,273, filed on Dec. 16, 1982, teaching a Print Head for Printing or Vector Plotting with a Multiplicity of Line Widths (Anderson; assigned to the common assignee herein). Anderson shows a multi-nozzle arrangement (a xe2x80x9cprimitivexe2x80x9d) for an 80-100 dpi raster/vector plotter with ink jet nozzles at selected points of a two-dimensional grid. However, while Anderson teaches a variety of useful primitive patterns (see e.g., FIGS. 1A-2B therein), the dot pattern is specifically limited to having only one nozzle on any given column in the grid by having only one nozzle in any given row or column. Selective firing is then directed depending on the plot to be created. A heavy interlacing of dots is required as demonstrated in FIGS. 4 and 5 therein.
Another problem with thermal ink-jet print heads is the phenomenon known as xe2x80x9cpuddling.xe2x80x9d An ink drop exiting an orifice will tend to leave behind minute amounts of ink on the nozzle plate surface about each orifice. As these puddles grow, surface tension between the puddle and an exiting ink drop will tend to attract the tail of the drop and change its trajectory. A change in trajectory means the drop will not hit its targeted pixel center, introducing, printing errors on the media. Tuning of nozzle plates is proposed by Allen et al. in U.S. Pat. No. 4,550,326 for Fluidic Tuning of Impulse Jet Devices Using Passive Orifices (assigned to the common assignee herein).
Another problem in ink-jet printing occurs at higher resolutions, for example, in multi-pass and bidirectional 300 dpi printing. Misaligned drops cause adverse consequences such as graininess, hue shift, white spaces, and the like. Normally, binary drops are deposited on the grid of square pixels such that drops overlap to a degree necessary to ensure no visible white spaces occur at the four corners of the target pixel (as taught by Trask, Doan, and Hickman, supra). As mentioned, ink usage is dramatically increased by these techniques. Moreover, print media line feed error is significant compared to drop size and, without multiple-drop or overlap between pixels, white banding between swaths occurs. Thus, each of these prior art inventions are using more ink than would be required if perfectly accurate trajectories of perfectly sized ink drops could be achieved.
Therefore, until a technological breakthrough to achieve such perfection is attained, there is still a need for improvement in thermal ink-jet print heads and methods of distribution of ink drops to achieve superior print quality, decreasing quantization effects and ink usage. The goal is to reduce the required luminance and color quantization levels of an ink-jet printing system for high fidelity without requiring higher dot placement printing resolution while also increasing data throughput.
In its basic aspects, the present invention provides an print head device for use in printing a pixel dot matrix on a print medium. The print head device includes: an array of drop generators, each of the drop,generators having a plurality of nozzles and the plurality of nozzles is configured such that each drop generator includes a set of nozzles in a predetermined layout providing a set of nozzles in each of the drop generators wherein as a drop generator traverses print medium target pixels as the print head is scanned across the medium, the nozzles in each set provide a distribution of ink droplets forming dots on the medium such that at least one of the dots formed on the medium from each set is substantially outside the target pixel.
Another basic aspect of the present invention is an ink-jet pen. The pen includes: a housing; at least one on-board ink reservoir within the housing, the reservoir containing at least one supply of ink of a predetermined chemical formulation; a print head fluidically coupled to the reservoir to receive a flow of ink therefrom; electrical contacts for connecting the print head to a hard copy apparatus print controller; the print head having a plurality of drop generators oriented in an array; each drop generator of the array having a plurality of nozzles arrayed about a geometric center point of the drop generator; each of the drop generators having at least one heating element connected to the electrical contacts; each of the nozzles having an ink entrance port proximate the heating element, the entrance port having an entrance port areal dimension; each of the nozzles having an exit orifice distal from the heating element for emitting ink drops onto an adjacently positioned print medium, the exit orifice having a predetermined exit orifice areal dimension less than an areal dimension of a pixel to be printed using the cartridge and less than the entrance orifice areal dimension and wherein the sum of the areal dimensions of the exit orifices in an array of nozzles is less than the areal dimension of a pixel.
In another basic aspect of the invention there is taught a method of distributing ink drops onto an adjacent print medium in order to form a dot matrix print on a grid of pixels wherein the dot matrix is manipulated selectively to form graphic art, images, and alphanumeric characters. The method includes the steps of:
scanning a print medium with at least one ink-jet pen in a first axial direction, X;
during the step of scanning,
simultaneously generating a plurality of ink drops in each drop generator of a drop generator array of an ink-jet print head of the ink-jet pen,
simultaneously firing sets of the simultaneously generated ink drops selectively at the grid of pixels such that each of the sets of ink drops form dots on the media, each of the dots having a size less than the size of a pixel, and each of the sets of ink drops being distributed in a pattern on or about a target pixel of the grid such that each of the drops of a set produces a dot having an area less than or equal to 1 divided by number-of-drops-per-set multiplied by the area of the target pixel areadotxe2x89xa6(1/n) * Pa where xe2x80x9cnxe2x80x9d is the number of orifices per drop generator and xe2x80x9cPaxe2x80x9d is the area of a pixel to be printed)
In yet another basic aspect the present invention provides for an ink-jet hard copy apparatus, having a housing, a scanning carriage, at least one pen mounted in the carriage, and a platen where swath printing operation is performed. The apparatus further provides for the pen having a housing; at least one on-board ink reservoir within the housing, the reservoir containing at least one supply of ink of a predetermined chemical formulation; a print head fluidically coupled to the reservoir to receive a flow of ink therefrom; electrical contacts for connecting the print head to a hard copy apparatus print controller; the print head having a plurality of drop generators oriented in an array; each drop generator of the array having a plurality of nozzles arrayed about a geometric center point of the drop generator; each of the drop generators having at least one heating element connected to the electrical contacts; and each of the nozzles having an ink entrance port proximate the heating element, the entrance port having an entrance port areal dimension, each of the nozzles having an exit orifice distal from the heating element for emitting ink drops onto an adjacently positioned print medium, the exit orifice having a predetermined exit orifice areal dimension less than the areal dimension of a pixel to be printed using the cartridge and less than the entrance orifice areal dimension and wherein the sum of the areal dimensions of the exit orifices in an array of nozzles is less than the areal dimension of a pixel, and each of the nozzles of each of the drop generators are oriented in a position rotated about a geometric center point of the drop generator with respect to an intersection of axes in a plane of a scan axis and a plane of a media motion axis such that dots are printed from each of the nozzles in adjoining pixels to a pixel which a drop generator is traversing, and each exit orifice has an exit orifice areal dimension sized to eject a droplet that will create a dot on a target media with an areal dimension less than or equal to an area calculated in accordance with a formula: 1 divided by the number of orifices per drop generator times the areal dimension of a pixel (Aeo=(1/n)*. Pa, where xe2x80x9cAeoxe2x80x9d is the exit orifice area, xe2x80x9cnxe2x80x9d is the number of orifice per drop generator, and xe2x80x9cPaxe2x80x9d is the area of a pixel to be printed)
It is an advantage of the present invention that it provides a method for lowering edge transition sharpness.
It is a further advantage of the present invention that it improves the imaging of luminance transition zones.
It is an advantage of the present invention that it achieves lower print graininess and smoother color transitions in the printing of mid-tone regions than is achieved using single orifice drop generators implementing the same dot placement resolution, without requiring increased printing resolution or number of multi-drop mode print levels.
It is an advantage of the present invention that it substantially eliminates the need for overlapping of printed dots to reduce quantization errors, deceasing the amount of ink needed to print an image.
It is an advantage of the present invention that it improves ink-jet print quality perception without increasing ink quantity per print.
It is an advantage of the present invention that it decreases graininess of an ink-jet print without reducing dye load in the ink.
It is another advantage of the present invention that it reduces the amount of water or other dye solvent deposited on the print media, thereby reducing both drying time and print media cockle effects.
It is another advantage of the present invention that nozzle dimensions are reduced, decreasing refill time (refill time is proportional to the capillarity force which is inversely proportional to exit orifice diameter) and increasing hard copy throughput proportionally.
It is another advantage of the present invention that reduced nozzle dimensions forming smaller ink drops requires less firing energy per drop from the heating element of the drop generator, improving thermal characteristics and print head life expectancy.
It is yet another advantage of the present invention that it increases life of the print head as heating element resistors are not required to fire as many times per pixel as in commercial multi-drop mode hard copy apparatus.
It is another advantage of the present invention that it improves print quality through reducing sensitivity to drop misalignment, decreasing sensitivity to trajectory errors caused by formation of puddles of ink around a nozzle""s exit orifice.
It is yet another advantage of the present invention that print quality is improved while using less ink by distributing a given drop volume, e.g., of a 600 dpi drop, over the area of a larger region, e.g., four quadrants of a 300 dpi pixel area, approximately one-quarter the saturation of the full dye load, lowering the density of the page by spreading less ink more evenly over the pixels.
It is still another advantage of the present invention that a multi-nozzle drop generator can be adapted to a variety of layout configurations such that resulting dots on the print media form more diffuse pixel fill, require less ink to print, and conceal drop misalignment errors, sheet feed errors, and trajectory errors.
It is still another advantage of the present invention that graphics and images require only single inks of primary colors to produce a range of hues formerly requiring multiple inks of primary colors using different dye loads or colorant formulations.
It is a further advantage of the present invention that it increases throughput by being adaptable to employing bidirectional scan printing.
It is a further advantage of the present invention that it is adaptable to a combination of orientations of each multi-nozzle drop generator such that printing errors, such as those caused by clogged nozzles or mis-firing drop generator nozzles, are masked in the print.
It is yet another advantage of the present invention that it eases the manufacturing tolerance requirement for nozzle-to-heating element alignment.
It is yet another advantage of the present invention that it can be retrofit to existing commercial ink-jet hard copy apparatus.
Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings.